The Home Secretary disclosed last week that immigration rules will be changed by the summer to ensure the “right to private and family life” can only be used to avoid deportation in “rare and exceptional cases”.

But the country’s most senior immigration judge has delivered a ruling in a landmark case which, experts say, reinforces the rights of immigrants who commit serious crimes to avoid deportation.

Mr Justice Blake, the president of the Immigration and Asylum Chamber, said a “settled migrant” could not be removed from the country unless there were “very serious reasons” to do so.

Having lived in the UK from a young age, or having a child or partner here, can strengthen a criminal’s claim to stay.

The judge has flagged up his ruling as a “reported determination”, which means that it will used by other judges to decide similar cases.

Meanwhile, a Conservative MP has warned that Mrs May’s plans to amend the rulebook for immigration officials may not go far enough, and new legislation may be required to ensure that foreign criminals can be returned to their homelands.

Officials from 47 European countries will meet in Brighton this week to debate plans put forward by the Government to reform the European Court of Human Rights, in an attempt to limit its interventions in the UK.

Mr Justice Blake’s decision came in the case of a foreign criminal who was convicted of drug dealing and burglary, but who later overturned a bid by the Home Office to deport him to Pakistan.

Shabaz Masih came to Britain in 1998, aged 10, with his family, who claimed asylum as members of the Christian minority in their homeland. By the age of 15 he was using Class A drugs.

In 2009, he was jailed for 50 months at Ipswich Crown Court for possessing Class A drugs with intent to supply and for another crime which involved burgling a house to steal two cars, including one which was driven away at high speed, and crashed.

Masih, from Haringey, north London, was part of a gang known as the “James Business”, distributing heroin and crack cocaine from a flat above an antiques shop in Ipswich.

Just before his arrest for the burglary Masih conceived a child with a British woman, Jade Millard, and their son was born in March 2009.

A report by probation officers at the time of Masih’s sentencing said he presented a high risk of reoffending and a medium risk to the public.

At the end of Masih’s jail term the Home Office began proceedings to remove him from Britain under laws which state that anyone jailed for 12 months or more is liable to automatic deportation.

He appealed and won his case, citing Article 8 of the European Convention on Human Rights which protects the “right to a private and family life”. The Home Office lodged a further appeal.

Hearing the new appeal, Mr Justice Blake ruled: “A first sentence of imprisonment, especially if it is as long as this one was, may have a rehabilitative effect on a young offender whose problems seemed to be linked with his abuse of drugs.”

He heard evidence that Masih, now 25, had “put crime ... behind him” and had been free of drugs since being released.

Mr Justice Blake refused the Home Office’s appeal, and said previous case law showed that a “settled migrant” who had “lawfully spent all or the major part” of their youth in this country could only be deported if there were “very serious reasons” to justify the steps.

The Sunday Telegraph’s End the Human Rights Farce campaign has highlighted cases where criminals have escaped deportation, often by claiming their rights under Article 8.

In one case, he ruled that deporting Rocky Gurung, a 22-year-old killer, to Nepal would breach his right to a family life, even though he was single, had no children and lived with his parents. The judgment was later quashed by Court of Appeal judges.

A Home Office spokesman said: “Too often Article 8 has been used by criminals to dodge deportation and by this summer the Government will have in place new immigration rules which will end this abuse.”